1. Field
The present invention relates to battery packs including secondary batteries used in electric cars or electric bicycles.
2. Description of the Related Art
Secondary batteries refer to batteries that may be rechargeable. The secondary batteries are the primary components for providing power to portable electronic appliances such as cellular phones, laptop computers, or camcorders. Also, secondary batteries are used as primary components for providing power to environment-friendly transportation devices such as electric cars or electric bicycles.
Unit cells of the secondary battery include an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode plate, a negative electrode plate, and a separator therebetween. The electrolyte is filled between the positive electrode plate and the negative electrode plate of the electrode assembly. Lithium ions contained in the electrolyte pass through the separator and move between the positive electrode plate and the negative electrode plate to thereby cause charging or discharging. During charging, electric energy is converted to chemical energy, and during discharging, chemical energy is converted to electric energy. During the energy conversion, heat is generated in the unit cells due to, for example, collision of lithium ions.
For high power output and large capacity of an electric bicycle or an electric car, a plurality of unit cells of secondary batteries may be stacked horizontally and/or vertically.
In a battery module, heat may be generated in the unit cells during charging and discharging and also due to changes in the external environment or due to external impact.
If the heat is not efficiently discharged from each of the unit cells, the unit cells may be over-heated and thus the charging or discharging performance may be degraded, and at worst, the battery module may explode. Meanwhile, if the ambient temperature of the battery module is low, the output power may be low.